The main process methods of aluminum alloy heat treatment are annealing, quenching and aging.
(1) Annealing
① Recrystallization annealing After holding for a period of time above the recrystallization temperature, air cooling is used to eliminate cold deformation strengthening of deformed workpieces and improve plasticity in order to continue forming processing.
② The purpose of low-temperature annealing is to eliminate internal stress and appropriately increase plasticity. It is usually air-cooled after heat preservation at 180～300℃.
③ Homogenization annealing The purpose is to eliminate component segregation and internal stress of ingots or castings, and improve plasticity. Usually, it is air-cooled after high temperature and long-term heat preservation.
(2) Quenching (Solution treatment) The aluminum alloy is heated to above the solution line and then quickly cooled, so that the second phase is too late to precipitate, and a supersaturated, unstable single α solid solution is obtained. The strength and hardness of the aluminum alloy after quenching are not high, and it has good plasticity.
(3) Aging The phenomenon in which the quenched aluminum alloy is heated at room temperature or low temperature for a period of time, and its strength and hardness increase significantly with time, and the plasticity decreases, which is called aging. The aging performed at room temperature is called natural aging; the aging performed under low temperature heating is called artificial aging. The essence of aging is that the second phase precipitates and grows from a supersaturated, unstable single α solid solution. Due to the different coherence degree of the second phase and the parent phase (α phase), the parent phase produces lattice distortion and strengthens. .
The aging strengthening effect of aluminum alloy is related to the heating temperature and holding time. The higher the aging temperature, the faster the aging speed. Each aluminum alloy has its best aging temperature and aging time. If the aging temperature is too high or the holding time is too long, the aluminum alloy will soften instead, which is called over-aging.
(4) After returning to the natural aging Al alloy, keep it at 200～250℃ for a short time (a few seconds to a few minutes), and then quench it with water, which can restore the mechanical and physical properties of the aging-strengthened Al alloy to the quenched state. This phenomenon is called regression. The Al alloy after the regression treatment can still be naturally aged, but its strength is reduced after each regression treatment, so the general number of regression treatments is limited to 3 to 4 times. Using the regression phenomenon, the riveting and repair of the aircraft can be carried out at any time.